Lithographic apparatus and device manufacturing method

ABSTRACT

An immersion lithographic projection apparatus has a liquid confinement structure configured to at least partly confine liquid to a space between a projection system and a substrate, the confinement structure having a buffer surface, when in use, positioned in close proximity to a plane substantially comprising the upper surface of the substrate and of a substrate table holding the substrate, to define a passage having a flow resistance. A recess is provided in the buffer surface, the recess, when in use, being normally full of immersion liquid to enable rapid filling of a gap between the substrate and substrate table as the gap moves under the buffer surface. The recess may be annular or radial and a plurality of recesses may be provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 10/948,749 filed on Sep. 24, 2004, now allowed, from whichbenefit is claimed under 35 U.S.C. §120. The entire contents of thisapplication is herein fully incorporated by reference.

FIELD

The present invention relates to a lithographic apparatus and a methodfor manufacturing a device.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein.

However, submersing the substrate or substrate and substrate table in abath of liquid (see, for example, U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate (the substrategenerally has a larger surface area than the final element of theprojection system). One way which has been proposed to arrange for thisis disclosed in PCT patent application WO 99/49504, hereby incorporatedin its entirety by reference. As illustrated in FIGS. 2 and 3, liquid issupplied by at least one inlet IN onto the substrate, preferably alongthe direction of movement of the substrate relative to the finalelement, and is removed by at least one outlet OUT after having passedunder the projection system. That is, as the substrate is scannedbeneath the element in a −X direction, liquid is supplied at the +X sideof the element and taken up at the −X side. FIG. 2 shows the arrangementschematically in which liquid is supplied via inlet IN and is taken upon the other side of the element by outlet OUT which is connected to alow pressure source. In the illustration of FIG. 2 the liquid issupplied along the direction of movement of the substrate relative tothe final element, though this does not need to be the case. Variousorientations and numbers of in- and out-lets positioned around the finalelement are possible, one example is illustrated in FIG. 3 in which foursets of an inlet with an outlet on either side are provided in a regularpattern around the final element.

When an immersion lithographic apparatus is used, bubbles in theimmersion liquid may cause manufacturing defects if those bubbles arepresent in the exposure field or the path of the projection beam duringexposure. Bubbles at or close to the substrate surface print as harddefects that are generally serious enough to render a device unusablewhile bubbles that are floating freely in the path of the projectionbeam can cause illumination non-uniformity and CD variation. Thepresence of more than a few bubbles per layer per substrate cantherefore reduce the yield of correctly functioning devicessubstantially.

SUMMARY

Accordingly, it would be advantageous, for example, to provide animmersion lithographic apparatus in which the generation of bubbles inthe immersion liquid is reduced.

According to an aspect of the invention, there is provided alithographic projection apparatus arranged to project a patterned beamof radiation from a patterning device, through a liquid, onto asubstrate, the apparatus comprising:

-   -   a projection system configured to project the patterned beam of        radiation onto the substrate; and

a liquid confinement structure configured to at least partly confine theliquid to a space between the projection system and the substrate,wherein the confinement structure has a buffer surface positioned, whenin use, in close proximity to a plane substantially comprising the uppersurface of the substrate and of a substrate table configured to hold thesubstrate, to define a passage having a flow resistance, the buffersurface comprising a recess, said recess when in use, being normallyfull of liquid to enable rapid filling of a gap between the substrateand the substrate table as the gap moves under the buffer surface.

According to an aspect of the invention, there is provided A devicemanufacturing method, comprising:

confining liquid to a space between a projection system of alithographic projection apparatus and a substrate using a liquidconfinement structure of the lithographic projection apparatus, theconfinement structure having a buffer surface positioned in closeproximity to a plane substantially comprising the upper surface of thesubstrate and of a substrate table holding the substrate, to define apassage having a flow resistance;

providing liquid in a recess in the buffer surface to enable rapidfilling of a gap between the substrate and the substrate table as thegap moves under the buffer surface; and

projecting a patterned beam of radiation, using the projection system,through liquid in the space onto the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system for use in a lithographicprojection apparatus;

FIG. 4 depicts another liquid supply system for use in a lithographicprojection apparatus;

FIG. 5 depicts a further liquid supply system for use in a lithographicprojection apparatus;

FIG. 6 depicts in plan the immersion hood of an embodiment of theinvention passing over the substrate edge gap;

FIGS. 7 to 9 depict in section the immersion hood of an embodiment ofthe invention passing over the substrate edge gap;

FIG. 10 depicts in plan the immersion hood of another embodiment of theinvention passing over the substrate edge gap; and

FIG. 11 depicts in section the immersion hood of another embodiment ofthe invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

-   an illumination system (illuminator) IL configured to condition a    radiation beam PB (e.g. UV radiation or DUV radiation).-   a support structure (e.g. a mask table) MT constructed to support a    patterning device (e.g. a mask) MA and connected to a first    positioner PM configured to accurately position the patterning    device in accordance with certain parameters;-   a substrate table (e.g. a wafer table) WT constructed to hold a    substrate (e.g. a resist-coated wafer) W and connected to a second    positioner PW configured to accurately position the substrate in    accordance with certain parameters; and-   a projection system (e.g. a refractive projection lens system) PL    configured to project a pattern imparted to the radiation beam PB by    patterning device MA onto a target portion C (e.g. comprising one or    more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structurecan use mechanical, vacuum, electrostatic or other clamping techniquesto hold the patterning device. The support structure may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam PB is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam PB passes through the projection system PL, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g. an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g. so as to position different target portions Cin the path of the radiation beam PB. Similarly, the first positioner PMand another position sensor (which is not explicitly depicted in FIG. 1)can be used to accurately position the mask MA with respect to the pathof the radiation beam PB, e.g. after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the mask table MT maybe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which form part of thefirst positioner PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

-   1. In step mode, the mask table MT and the substrate table WT are    kept essentially stationary, while an entire pattern imparted to the    radiation beam is projected onto a target portion C at one time    (i.e. a single static exposure). The substrate table WT is then    shifted in the X and/or Y direction so that a different target    portion C can be exposed. In step mode, the maximum size of the    exposure field limits the size of the target portion C imaged in a    single static exposure.-   2. In scan mode, the mask table MT and the substrate table WT are    scanned synchronously while a pattern imparted to the radiation beam    is projected onto a target portion C (i.e. a single dynamic    exposure). The velocity and direction of the substrate table WT    relative to the mask table MT may be determined by the    (de-)magnification and image reversal characteristics of the    projection system PL. In scan mode, the maximum size of the exposure    field limits the width (in the non-scanning direction) of the target    portion in a single dynamic exposure, whereas the length of the    scanning motion determines the height (in the scanning direction) of    the target portion.-   3. In another mode, the mask table MT is kept essentially stationary    holding a programmable patterning device, and the substrate table WT    is moved or scanned while a pattern imparted to the radiation beam    is projected onto a target portion C. In this mode, generally a    pulsed radiation source is employed and the programmable patterning    device is updated as required after each movement of the substrate    table WT or in between successive radiation pulses during a scan.    This mode of operation can be readily applied to maskless    lithography that utilizes programmable patterning device, such as a    programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets IN oneither side of the projection system PL and is removed by a plurality ofdiscrete outlets OUT arranged radially outwardly of the inlets IN. Theinlets IN and OUT can be arranged in a plate with a hole in its centerand through which the projection beam is projected. Liquid is suppliedby one groove inlet IN on one side of the projection system PL andremoved by a plurality of discrete outlets OUT on the other side of theprojection system PL, causing a flow of a thin film of liquid betweenthe projection system PL and the substrate W. The choice of whichcombination of inlet IN and outlets OUT to use can depend on thedirection of movement of the substrate W (the other combination of inletIN and outlets OUT being inactive).

Another immersion lithography solution with a localized liquid supplysystem solution which has been proposed is to provide the liquid supplysystem with a seal member which extends along at least a part of aboundary of the space between the final element of the projection systemand the substrate table. The seal member is substantially stationaryrelative to the projection system in the XY plane though there may besome relative movement in the Z direction (in the direction of theoptical axis). A seal is formed between the seal member and the surfaceof the substrate. In an embodiment, the seal is a contactless seal suchas a gas seal. Such a system with a gas seal is disclosed in U.S. patentapplication Ser. No. 10/705,783, hereby incorporated in its entirety byreference, and shown in FIG. 5.

As shown in FIG. 5, the reservoir 10 forms a contactless seal to thesubstrate around the image field of the projection system so that liquidis confined to fill a space between the substrate surface and the finalelement of the projection system. The reservoir is formed by a sealmember 12 positioned below and surrounding the final element of theprojection system PL. Liquid is brought into the space below theprojection system and within the seal member 12. The seal member 12extends a little above the final element of the projection system andthe liquid level rises above the final element so that a buffer ofliquid is provided. The seal member 12 has an inner periphery that atthe upper end preferably closely conforms to the shape of the projectionsystem or the final element thereof and may, e.g., be round. At thebottom, the inner periphery closely conforms to the shape of the imagefield, e.g., rectangular though this need not be the case.

The liquid is confined in the reservoir by a gas seal 16 between thebottom of the seal member 12 and the surface of the substrate W. The gasseal is formed by gas, e.g. air or synthetic air or N₂ or an inert gas,provided under pressure via inlet 15 to the gap between seal member 12and substrate and extracted via first outlet 14. The overpressure on thegas inlet 15, vacuum level on the first outlet 14 and geometry of thegap are arranged so that there is a high-velocity gas flow inwards thatconfines the liquid.

In European Patent Application No. 03257072.3, the idea of a twin ordual stage immersion lithography apparatus is disclosed. Such anapparatus is provided with two tables for supporting a substrate.Leveling measurements are carried out with a table at a first position,without immersion liquid, and exposure is carried out with a table at asecond position, where immersion liquid is present. Alternatively, theapparatus has only one table.

The liquid supply system IH, also referred to as the immersion hood, isshown schematically in plan in FIG. 6 and in vertical cross-section inFIG. 7 to 9, approaching the gap WEG between the substrate W and theupper surface of the substrate table WT. The immersion hood IH may bethe same as that described above in relation to FIG. 5, save asdescribed below, and a further description of the common aspects isomitted for brevity.

As shown in FIGS. 7 to 11, the substrate is placed on a burl plate BP(also variously referred to as a pimple plate, substrate holder orchuck) in a recess (sometimes referred to as a pothole) in the substratetable. To allow for variation in the size of the substrate and in theplacement of the substrate, the pothole is a little larger than thesubstrate. Thus when the substrate is in position, there is a narrow gaparound the edge of the substrate, which may have a width of about 0.5 to1 mm and a similar depth.

As can be most clearly seen in FIGS. 7 to 9, the seal member or liquidconfinement structure 12 rides close to the upper surface of thesubstrate/substrate table so that a narrow gap is formed in region D toreduce leakage of the immersion fluid. The immersion fluid is furtherconfined by a two-phase extraction passage 23, a gas extraction passage22 and a gas supply passage 21 which forms a gas knife to drive inwardlyany liquid remaining on the surface of the substrate W. A single(liquid) phase extractor may also be used in place of the two-phaseextractor.

In FIG. 6, the region where immersion liquid (e.g. ultra pure water) ispresent is shown with vertical hatching. In a central area encompassingthe exposure field, shown with darker hatching, the immersion liquidcompletely fills the space between the substrate W and projection systemPL so there is a considerable volume of liquid. Around that there is anannular region, shown in lighter hatching, where there is only a thinlayer of immersion liquid, sandwiched in a narrow gap between the sealmember 12 and the substrate W. The narrowness and width of this gapserve to confine the immersion liquid in the central area and reduceleakage. This region is referred to as the buffer. The buffer may insome embodiments, where the gap between the bottom of the seal member 12and the substrate is small, function to damp vibrations of the substratetable WT and seal member 12 and therefore may also be referred to as thedamper. In embodiments where the gap between the seal member andsubstrate is large, the buffer region may have little or no dampingeffect.

However, the volume of liquid in the buffer and the flow rate through itmay be insufficient to quickly fill the substrate edge gap as theimmersion hood passes over it. Thus air (or gas) can be left in thesubstrate edge gap WEG when the central area of the immersion hoodpasses over it. The greater volume of liquid in this area providesliquid to fill the gap but the displaced air (or gas) may be asignificant source of bubbles in the central region. These bubbles maycause printing defects before they are removed from the exposure field.

According to an embodiment of the invention therefore, a recess 30(which in this embodiment is annular but may be another shape) isprovided in the underside of the seal member 12, outside the centralarea and in the buffer region. The recess 30 is normally full of theimmersion liquid and functions to limit or prevent bubbles by enablingfaster filling of the substrate edge gap WEG. This occurs in two ways.Firstly, as shown in FIGS. 8 and 9, the recess 30 provides an additionalvolume of liquid that can flow directly into the substrate edge gap andsecondly, the recess provides a flow path enabling the immersion liquidto flow peripherally (in this case, circumferentially) to fill the gap,as shown in FIG. 6. At the same time however, the recess does notsubstantially reduce the radial flow resistance provided by the bufferregion. In an embodiment, the recess has a volume comparable to thevolume of the substrate edge gap to be filled, which can be achieved ifthe vertical cross-sectional area is from a half to twice that of thesubstrate edge gap. The liquid may be provided at an overpressure in therecess 30, to increase the speed of filling. With this embodiment, it ispossible to arrange that the substrate edge gap will be filled within afew milliseconds, preventing or strongly limiting gas escaping into theexposure field.

Although the recess is shown as extending completely around theperiphery (e.g., a complete annulus), it may be interrupted in one ofmore places provided the recess has sufficient volume and length toenable filling of the substrate edge gap in the desired time. Also, ifthe immersion hood will only approach the substrate edge in a limitednumber or range of directions, the recess may be provided only wherenecessary and not all around the seal member.

Another embodiment, that is a variation of the first embodiment, isshown in FIG. 10. In this embodiment, in place of a peripheral (e.g.,annular) recess 30, a plurality, in this case six, of channels 31 (inthis case, extending radially although they may arranged in otherdirections as appropriate) are provided. The channels are recessed intothe underside of the seal member 12 and extend outward from the centralreservoir. The dimensions—depth, width and length—and number of thechannels is selected to provide a flow path from the reservoir to thesubstrate edge gap WEG that is sufficient to fill the gap as fast asrequired, while at the same time minimizing leakage of the immersionfluid. Leakage can be prevented by ensuring that the channels do notextend as far as the one- or two-phase extraction passage 23. A suitablenumber of recesses is from 4 to 8. In an embodiment, the geometry of thechannels 31 is such that the substrate edge gap WEG will only be filledfrom one channel at a time, when approached from certain directions thatwill be used in production. For this reason, in an embodiment, the endsof the recesses all lie on a circle.

A further embodiment of the invention is shown in FIG. 11. Thisembodiment is essentially the same as the embodiments described above,save that the underside of the seal member 12 is provided with a sharp,hydrophilic edge 31 to control the position of the meniscus 32 of theimmersion liquid. (In this context hydrophilic is used to mean that thematerial of the edge has a contact angle to the immersion liquid of lessthan 90°, whether or not the immersion liquid is water.) The sharp edge31 is conveniently positioned at the edge of the buffer region D andlimits or prevents the bubbly liquid-gas mixture that is present outsidethe buffer region from passing into the buffer region.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm).

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, including refractiveand reflective optical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described.

The present invention can be applied to any immersion lithographyapparatus, in particular, but not exclusively, those types mentionedabove.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus arranged to project a patterned beam ofradiation from a patterning device, through a liquid, onto a substrate,the apparatus comprising: a projection system configured to project thepatterned beam of radiation onto the substrate; and a liquid confinementstructure configured to at least partly confine the liquid to a spacebetween the projection system and the substrate, wherein a surface ofthe confinement structure has a buffer surface positioned, when in use,in close proximity to a plane substantially comprising the upper surfaceof the substrate and of a substrate table configured to hold thesubstrate, to define a passage having a flow resistance, and the surfaceof the confinement structure has a recess and a sharp edge radiallyoutward or at the edge of the buffer surface, wherein the sharp edgeforms an acute angle from a horizontal plane of the upper surface of thesubstrate.
 2. The lithographic apparatus of claim 1, wherein the sharpedge is radially outward of the recess.
 3. The lithographic apparatus ofclaim 1, wherein the recess, in use, is normally full of liquid toenable rapid filling of a gap between the substrate and the substratetable as the gap moves under the buffer surface.
 4. The lithographicapparatus of claim 1, wherein the recess has a volume that issubstantially equal to a volume of a gap between the substrate and thesubstrate table.
 5. The lithographic apparatus of claim 1, wherein asurface of the sharp edge forms, in use, a contact angle with respect tothe liquid of less than 90 degrees.
 6. The lithographic apparatus ofclaim 1, wherein the recess has a cross-sectional area in the range offrom 0.5 to 2 times the nominal cross-sectional area of a gap betweenthe substrate and the substrate table.
 7. A device manufacturing method,comprising: confining liquid to a space between a projection system of alithographic projection apparatus and a substrate using a liquidconfinement structure of the lithographic projection apparatus, asurface of the confinement structure having a buffer surface positionedin close proximity to a plane substantially comprising the upper surfaceof the substrate and of a substrate table holding the substrate, todefine a passage having a flow resistance, and the surface of theconfinement structure having a recess and a sharp edge radially outwardor at the edge of the buffer surface, wherein the sharp edge forms anacute angle from a horizontal plane of the upper surface of thesubstrate; and projecting a patterned beam of radiation, using theprojection system, through liquid in the space onto the substrate. 8.The method of claim 7, wherein the sharp edge is radially outward of therecess.
 9. The method of claim 7, further comprising providing liquid inthe recess to enable rapid filling of a gap between the substrate andthe substrate table as the gap moves under the buffer surface.
 10. Themethod of claim 7, wherein the recess has a volume that is substantiallyequal to a volume of a gap between the substrate and the substratetable.
 11. The method of claim 7, wherein a surface of the sharp edgeforms, in use, a contact angle with respect to the liquid of less than90 degrees.
 12. The method of claim 7, wherein the recess extendssubstantially radially from the space.
 13. A lithographic projectionapparatus arranged to project a patterned beam of radiation from apatterning device, through a liquid, onto a substrate, the apparatuscomprising: a projection system configured to project the patterned beamof radiation onto the substrate; and a liquid confinement structureconfigured to at least partly confine the liquid to a space between theprojection system and the substrate, wherein the confinement structurehas a buffer surface and a sharp edge, the buffer surface and the sharpedge being positioned, when in use, in close proximity to a planesubstantially comprising the upper surface of the substrate and of asubstrate table configured to hold the substrate, to define a passagehaving a flow resistance, the sharp edge configured to control theposition of a meniscus of the liquid located between the buffer surfaceand the plane, wherein a surface of the sharp edge has a contact anglewith respect to the liquid of less than 90 degrees and the sharp edgeforms part of a periphery of an aperture of the confinement structure.14. The lithographic apparatus of claim 13, wherein the sharp edge ishydrophilic.
 15. The lithographic apparatus of claim 13, wherein thesharp edge is positioned at an outer edge of the buffer surface.
 16. Thelithographic apparatus of claim 13, wherein the sharp edge is configuredto limit, prevent, or both limit and prevent, a mixture of gas andliquid present outside a buffer region defined by the buffer surfacefrom passing into the buffer region.
 17. The lithographic apparatus ofclaim 13, further comprising a gas knife configured to drive liquidtoward an optical axis of the projection system.
 18. The lithographicapparatus of claim 17, wherein the gas knife comprises a passage toextract liquid, a passage to extract substantially only gas, and apassage to supply gas.
 19. The lithographic apparatus of claim 17,wherein the sharp edge is located inward, relative to an optical axis ofthe projection system, of the gas knife.